1. Field of the Invention
The present invention relates to a soft X-ray microscope comprising a soft X-ray source for emitting soft X-rays, a condenser lens for projecting the soft X-rays emitted from the soft X-ray source onto a specimen, an objective lens for focusing soft X-rays emanating from the specimen onto a predetermined position, and a soft X-ray detector arranged at said predetermined position for detecting the soft X-rays focused by said objective lens.
2. Related Art Statement
It has been well known that many elements have their specific absorption edges situating within a soft-X ray region generally of a wavelength longer than several angstroms. Therefore, by suitably selecting a wavelength of soft-X rays, it is possible to effect an observation with high resolution and high contrast without performing a preparatory operation for specimens. Generally, a wavelength region of the soft X-ray extends from 2 .ANG. which is the longest wavelength of the hard X-rays to 1000 .ANG. which is the shortest wavelength of the vacuum ultraviolets, so that the wavelength region of the soft X-rays partially overlaps with a wavelength region from 300 to 1000 .ANG. of the extreme ultraviolets.
For instance, within a so-called water window region between the K.alpha. absorption edge of carbon (44 .ANG.) and the K.alpha. absorption edge of oxygen (23.7 .ANG.), the absorption of water and protein differ from each other, and thus biological specimens having substantially no contrast and color under the visible light can be observed without performing any particular preparatory treatment such as dyeing. That is to say, biological specimens can be observed under living condition. Moreover, the L.alpha. absorption edge of silicon which has been widely used as a semiconductor material is 126- and that of aluminum which has been also widely used as an electrode wiring material in a semiconductor device is 169.8 .ANG., so that if the observation is carried out by using soft X-rays having wavelengths which are slightly shorter than said absorption edges respectively, distributions of these materials in the semiconductor device can be observed with a high resolution.
Due to the above mentioned facts as well as the development in ultra fine machining technique, there have been developed various soft X-ray microscopes using soft X-rays.
In the soft X-ray microscope, in order to remove or suppress undesired absorption of soft X-rays due to the air, various optical systems constituting the soft X-ray microscope are usually arranged within a vacuums chamber.
The soft X-ray microscope includes various optical systems such as condenser lens and objective lens. There have been proposed several optical systems such as the Schwaltzschild optical system using reflection surfaces having a multiple coating for revealing a high reflection for soft X-rays having a given wavelength, the Wolter optical system utilizing the total reflection and the zone plate optical system utilizing the diffraction.
Upon inspecting a specimen with the aid of the soft X-ray microscopes, the specimen has to be aligned with respect the optical axis by moving the specimen in a direction perpendicular to the optical axis such that a desired portion of the specimen can be inspected and further the specimen has to be moved in a direction of the optical axis with respect to the objective lens to effect the focus adjustment. The alignment of the specimen and focus adjustment are carried out while the specimen is observed under the soft X-rays. That is to say, the alignment and focus adjustment are performed also in the vacuum, so that these operations could not be performed efficiently.
Furthermore, in case of observing biological specimens, it is desired to reduce a dose rate of soft X-rays as far as possible. That is to say, it is desired to prevent the specimen form being subjected to an unnecessary irradiation with the soft X-rays.
Usually a soft X-ray source is formed by a synchrotron radiation (SR) source and laser plasma source. The SR X-ray source is very large in size and quite expensive in cost, so that a plurality of users commonly utilize the SR source, and therefore it is not always possible to utilize the SR source for the alignment and focus adjustment at will. At any rate, it is desired to effect the alignment and focus adjustment without using the SR source. Further, the laser plasma source can generates soft X-rays only in a pulsatory manner having a repetition frequency of, for instance 10 Hertzs, so that during the alignment and focus adjustment, the image of the specimen can be seen in a stroboscopic manner and thus the alignment and adjustment could not be performed easily.
Therefore, it has been required an X-ray microscope comprising the visible light observing optical system by means of which the alignment and focus adjustment can be performed within the vacuum condition without using the soft X-ray source.
In order to avoid the above mentioned inconvenience, there have been proposed soft-X ray microscopes in which the alignment and focus adjustment can be performed in the atmosphere by incorporating a visible light observing optical system. For instance, in Japanese Patent Laid-open Publications Kokai Sho 64-3600 and Kokai Hei 3-282300, there are proposed X-ray microscopes having the visible light observing optical systems installed therein.
FIG. 1 is a schematic cross sectional view showing a soft X-ray microscope disclosed in the above mentioned Kokai Hei 3-282300. The soft X-ray microscope comprises a soft X-ray source 1 for emitting soft X-rays, a condenser lens 2 formed by the Schwaltzschild optical system, and a vacuum chamber 3 in which the X-ray source 1 and condenser lens 2 are installed. The soft X-rays emitted from the X-ray source 1 are projected onto a specimen 4 under inspection by means of the condenser lens 2. A reference numeral 5 denotes an objective lens formed by the Schwaltzschild optical system having the same construction as that of the condenser lens 2. A reference numeral 6 denotes a soft X-ray detector and a reference numeral 7 represents a soft X-ray filter for cutting off radiation components having longer wavelengths than that of the soft X-rays. These elements 5, 6 and 7 are also installed within the vacuum chamber 3 together with the specimen 4. Soft X-rays emanating from the specimen 4 are focused onto the detector 6 by means of the objective lens 5. In this manner, the objective lens 5 constitutes an enlargement optical system. The soft X-ray detector 6 is connected to a signal processing circuit and an image signal produced by this circuit is supplied to a monitor to display a visible image of the specimen on the monitor.
In addition to the above explained soft X-ray microscope system, there is further provided a visually observing optical system for inspecting the image of the specimen under the visible light. That is to say, a visible light source 11 is arranged outside the vacuum chamber 3 and a transparent window 12 is provided in a wall of the vacuum chamber. Within the vacuum chamber 3, there are arranged first and second prisms 13 and 14. The first prism 13 is arranged movably to be selectively inserted into an optical path between the X-ray source 1 and the condenser lens 2, and the second prism 14 is also arranged movably to be selectively inserted into an optical path between the objective lens 5 and the filter 7. The movement of these prisms are schematically depicted by double headed arrows.
When the first and second prisms 13 and 14 are placed into the optical paths shown in FIG. 1, visible light emitted by the visible light source 11 is made incident upon the first prism 13 and is then reflected by the prism along the optical path along which the soft X-rays are made incident upon the condenser lens 2. The visible light emanating from the specimen 4 is focused on an image plane with is conjunction with the X-ray detector 6 by means of the second prism 14 to form a visible image of the specimen 4. Then, this visible image is observed by means of an eyepiece 15 provided in the wall of the vacuum chamber 3.
The condenser lens 2 and objective lens 5 are formed by the Schwaltzschild optical system includes multiple coatings which have a large reflectance not only for the soft X-rays but also for the visible light, and therefore the condenser lens 2 and objective lens 5 can be used as the condenser lens and objective lens, respectively of the visually observing optical system.
When the first and second prisms 13 and 14 are inserted into the optical path of the soft X-ray microscope optical system and the visible light source 11 is lit on, the visible image of the specimen 4 can be observed. In this case, in the known soft X-ray microscope, the visible light observing optical system is constructed as the bright field microscope which can inspect the contrast and color the specimen. That is to say, in the known soft X-ray microscope, the contrast and color of the specimen are of converted into the visible image.
In general, biological specimens have very low contrast, so that it is difficult to observe visible images of the biological specimens by means of the above mentioned known visually observing optical system. Therefore, the specimens are usually dyed with suitable dyeing agents in a preparatory step. As explained above, the soft X-ray microscope has an advantage that the specimen can be observed without performing the preparatory step. Therefore, in the known soft X-ray microscope, when a specimen has a low contract or has substantially no color, the alignment of the specimen and focus adjustment have to be carried out by observing the soft X-ray image of the specimen. This results in an undesired increase in the dose rate of soft X-rays.
Moreover, for some specimens it is required to observe them with a higher resolution than that can be attained by the visually observing optical system. Also in this case, the specimen has to be observed under the soft X-rays.
As explained above, in the known soft X-ray microscope having the visually observing optical system, the specific advantages of the soft X-ray microscope could not be fully attained.